Turns out stacking + trade makes clerks pretty good. Baseline they produce next to nothing, especially on sub-optimal worlds, but with a bunch of bonuses stacked they do alright.
Thrifty is key, though. It's 25% trade bonus, and unlike all other bonuses it's multiplicative with others. For example, say you have fanatic xenophile (+20% trade) and 10% bonus from stability and +10% bonus from the diplomacy tradition. So a trader gives you 2*1.4=2.8. Throw in Thrifty, and you get 2*1.25*1.4=3.5.
Even if you aren't focusing on trade, the extra 25% trade production on jobs is really nice. Thrifty is a really good trait.
The way this works make traits like Intelligent kind of sucky in comparison long-term. It's a 10% bonus that just gets added in to other bonuses instead of multiplied. And there is like +70% bonuses available from traits and the + research techs. I have an Intelligent worker in a lab next to one who isn't Intelligent and they're producing 7.6 and 7.2.
With Thrifty and some + trade bonuses you can do silly stuff like turning a world into nothing but city districts and commerce megaplexes and pulling in 1100+ trade value. That's only really viable on the capital area, though - can you imagine trying to manage piracy on trade routes that had a bunch of 1100+ trade value worlds? You would have to have huge war fleets doing patrols.
Using ringworlds as trade worlds is actually pretty good. I've heard people say they were useless, but if you fill them up with commerce megaplexes and city districts and farms you can produce a huge surplus of food and energy and consumer goods, then focus all your other worlds on producing minerals and alloys and the rare resources. But you would want to make sure any ringworld for this purpose is built within 6 jumps of your capital starbase to avoid piracy.